An electronic microprocessor-based unit may be used to control an actuator used in a variety of automated valve applications. Such a unit is commonly referred to as an electronic positioner, or simply a “positioner,” or as a servo card, or simply a “servo.” An AC actuator is an electromechanical device that uses a motor, such as an AC split phase motor or even a three-phase motor, to rotate an output shaft that can be mechanically coupled to a valve, thus allowing the actuator to open and close the valve. A DC actuator uses a DC motor. Some actuators may use the motor to move a linear rod back and forth for special type applications.
Most valves used to control the flow of fluids or gases through piping naturally tend to have a rotation of 90°, or one quarter of a circle. Hence, the name “quarter turn actuator” is widely used and recognized. In most applications, 0° is identified with the closed position, while 90° is identified with the full open position. From a control view point, the valve is commonly described in terms of percent open, where 0% corresponds to the closed position and 100% corresponds to the full open position, regardless of the number of degrees between the open and closed position. Quarter turn actuators are typically rated for their speed, where the rating defines the number of seconds the actuator requires to move from 0° to 90°. Most actuators fall within a range between 2 seconds and 90 seconds.
Referring to FIG. 1, an exemplary AC split phase motor 10 has been illustrated. It includes motor windings 12 and 14. The motor winding 12 provides for clockwise motion of the motor 10 while the motor winding 14 provides for counterclockwise motion of the motor 10. With respect to a valve (not shown), the motor winding 12 provides motor motion to open the valve and the motor winding 14 provides motion to close the valve. First ends of each of the windings are connected to a first AC power line 16. As shown, the first AC power line 16 is a neutral line and is referred to as “motor neutral” or “motor common.” A control switch 18 may then be used to operate the valve by connecting a second AC power line 20 to the appropriate winding.
Limit switches 22 and 24 are typically used to disconnect power from the motor 10 when the valve has reached its end of travel in order to avoid a continuous stall condition on the motor 10. A continuous stall condition may cause the motor windings 12 and 14 to overheat and permanently damage the motor 10. The limit switches 22 and 24 are typically actuated by cams mounted on an output shaft of the motor 10. The cams are positioned to turn the limit switches 22 and 24 off at a desired point of rotation. In other words, the limit switches 22 and 24 are turned off when the valve is completely open or closed.
AC motors may be equipped with a thermal switch 26. The thermal switch 26 disconnects the first AC power line 16 when a specific motor temperature, such as 200° F., is reached, and then reconnects the first AC power line 16 when the motor 10 has cooled. This feature safeguards against adverse conditions such as a motor stall in the normal operating range, a motor failure, and/or excessive cycling of the actuator.
Some actuators implement torque switches to detect a motor stall condition. The thermal switch 26 is primarily intended as a safety feature to avoid a fire hazard. However, regular tripping of the thermal switch 26 can damage the motor components leading to significantly reduced motor life. In contrast, torque switches can turn off the motor 10 before excessive heating occurs. A torque switch is implemented for each direction, open and close, so that if one of the torque switches turns off the motor in its associated direction, the other motor winding can be operated. If the motor 10 is able to move freely in the other direction, the torque switch is reset, thus allowing operation in the first direction. Electrically, each torque switch is wired in series with the limit switch for a given winding, so the limit switches 22 and 24 electrically represent both a limit switch and a torque switch, where either a limit position or a torque trip function will disconnect the motor winding.
Still referring to FIG. 1, a motor capacitor 28 creates a phase shift between the powered winding and the unpowered winding. The phase shift in the unpowered winding generates a magnetic field that works in conjunction with the primary magnetic field in the powered winding that causes the motor 10 to rotate in a specific direction. When the other winding is powered, the phase shift causes the magnetic polarity between the windings to switch, thus causing the motor 10 to rotate in the opposite direction. When the capacitance of the capacitor 28 is increased, the phase shift also increases, thus creating a greater magnetic differential between the windings. This results in higher torque output from the motor 10. While larger capacitance is commonly used to obtain higher torques, the increased capacitance also allows a higher current to flow through the unpowered winding. This increased current results in additional heating of the motor 10, and consequently motor specifications are usually derated from 100% duty to levels as low as 25% duty.
When power is initially applied to a given winding, an in-rush current is generated that is equal to the AC voltage divided by motor winding resistance. Once the motor begins rotating, its motion through the magnetic fields generates a counter EMF that dramatically reduces the current draw. The unpowered winding also adds to the in-rush current by drawing current through the motor capacitor 28, so when larger capacitance is used to increase torque, the in-rush current also increases. In-rush currents typically range from 2 to 3 times the normal running current and typically last for 100 milliseconds. The in-rush current causes a dramatic heating effect compared to normal running currents, and consequently motors are commonly rated for a maximum number of starts per hour. For example, a 100% duty motor may be rated for a maximum of 12,000 starts per hour.
If the motor 10 mechanically stalls, the motor current will naturally increase to the value of the in-rush current since the motor 10 is not rotating through the magnetic fields. While torque switches safeguard against most stall scenarios, an occasional failure in typical applications occurs when a valve is restricted by debris, which allows the actuator to move within a small range (e.g. a few degrees). This would allow an erratic control signal to constantly oscillate between stall conditions in both directions. The resulting combination of stall currents and inrush currents eventually overheat the motor 10, thus tripping the thermal switch 26.
Electronic Positioner Basics
In order to control a motor, such as the illustrated AC split phase motor electronically, the control switch 18 is replaced with an electronically controlled switching device. One approach is to replace the control switch 18 with a relay that can be controlled by electronic circuits. However, a relay may switch at a random point of time during the AC sine wave. When the relay contact applies power to the motor winding near the peak of the sine wave, the sudden change in voltage to the motor winding generates a significant electrical transient, which can cause electrical interference with the circuits controlling the relay as well as other electronic equipment in the vicinity. Additionally, the mechanical nature of a relay places limits on the number of switching cycles. Typically, the number of switching cycles is less than 1,000,000. The transients generated during switching causes undesirable electrical arcing between the relay contacts, which burns the contact surfaces and gradually degrades the relay. This results in an electrical cycle limit that is typically 1/10 of the mechanical cycles.
A better solution to controlling the motor is to replace the control switch 18 or relay with solid state devices which do not have moving mechanical parts, thus eliminating mechanical wear and the undesirable arcing. While a variety of solid state devices and circuits can he used, the most common devices used are triacs 30 and 32 as shown in FIG. 2. The triac 30 includes a primary triac Q1 and a secondary triac Q1a. The triac 32 includes a primary triac Q2 and a secondary triac Q2b. The secondary triacs Q1a and Q2b are used to gate the primary triacs Q1 and Q2, respectively, thereby turning the primary triacs Q1 and Q2 on and off. The secondary triacs Q1 a and Q2b are optoelectronic devices that provide electrical isolation between the motor circuit and the control circuit in the same manner that a relay coil is isolated from its contacts. Additionally, control LEDs LED1 and LED2 may be controlled by a low voltage, low powered device such as a microprocessor. Resistors 34 and 36 in the secondary triac circuits are used to limit current through the gates of the primary triacs Q1 and Q2.
A common practice that further enhances triac control is the use of a zero-crossing circuit that prevents on or off switching of the primary triacs Q1 and Q2 unless the AC line voltage is at a zero voltage point. This dramatically reduces the electrical transients generated when power is applied to the motor 10 at random times. Optically coupled triac devices have long been available with such zero-crossing circuits integrated within the device and are commonly implemented in positioner designs. This allows random switching of the control LEDs LED1 and LED2 without causing random switching of the primary triacs Q1 and Q2.
While zero-crossing controlled triacs resolve random switching problems, the limit switches 22 and 24, torque switches, thermal switch 26, or any other power disruption of the AC line voltage can cause “random switching” of power to the motor 10. These events generate transients that may damage the triac circuits 30 and 32. To prevent damage to the triacs 30 and 32, snubber circuits (not shown) may be used.
A means to control the positioner is required in order to turn the motor 10 on and off to achieve a specific percent-open position. Automated control systems may provide command signals that are either analog or digital. Common analog signals used are 0-10V, 0-5V, 1-5V, or 4-20 mA, where the particular signal used represents 0 to 100% open. Digital signals may take the form of pulse width modulation, frequency modulation, or one of many forms of data communications.
Regardless of what type of signal is used, the signal is interpreted as a percent-open command. In order to turn the motor 10 off at the desired position as dictated by the command signal, the positioner monitors the position of the actuator's output shaft. Monitoring the output shaft position may be achieved by mechanically coupling a feedback potentiometer to the output shaft. Most actuators use a set of gears to couple the feedback potentiometer by mounting one gear to the output shaft, which in turn rotates a second gear mounted to the potentiometer shaft. Since potentiometers and actuators have a finite rotating range, the potentiometer rotation needs to be aligned with the output shaft rotation. That is, when the valve position is at 50% open, the potentiometer wiper is ideally at 50% resistance. Alignment is accomplished by tightening the gears to their respective shafts when the valve is 50% open and the potentiometer is set to 50%.
Referring now to FIG. 3. a positioner 40 applies an excitation voltage +V (usually 10V or less) to a feedback potentiometer 42 and measures the voltage on the potentiometer's wiper 44 which is proportional to the angular position of the output shaft. To position the valve to a desired percent-open, the positioner 40 compares a feedback signal 46 from the feedback potentiometer 42 to the command signal 48 and determines whether to turn on the open motor winding or the close motor winding, and then ultimately turns the motor 10 off when the command signal 48 matches the feedback signal 46. Analog positioners achieve this by scaling the command signal 48 and/or the feedback signal 46 using adjustable amplifiers (scaling amplifiers) with adjustable offsets, so that both signals produce equal voltages at 0% and 100% open. The offset adjustment normally dictates the 0% value and is referred to as the zero setting. The gain of the amplifier dictates the 100% value and is referred to as the span setting. Digital positioners perform the same task by converting the command signal 48 and the feedback signal 46 to numerical values using an Analog-to-Digital (A/D) converter circuit. Once in numerical form, a microprocessor can set zero using addition operations, set span using multiplication operations, and then logically compare the scaled numerical values. For digital positioners, comparators 50 and 52 represent a logic operation rather than actual circuits.
In order to function correctly, the motor 10 and feedback potentiometer 42 must be wired to the positioner 40 in a particular manner. In most applications, when the motor 10 moves the valve toward the open position, the wiper 44 of the feedback potentiometer 42 will move toward the +V terminal, which increases the voltage measured on the wiper 44. In this case, commonly called “forward acting,” the comparator 50 will turn on the open motor winding whenever the feedback signal 46 is less than the command signal 48. Likewise, comparator 52 will turn on the close motor winding whenever the feedback signal 46 is greater than the command signal 48. The eventual relationship between the valve motion and the feedback signal 46 can be inverted by a variety of conditions such as mounting orientation of the feedback potentiometer 42, the coupling mechanism to the feedback potentiometer 42, and mechanical couplings between the actuator and valve. Consequently, some applications require “reverse acting.” For reverse acting, when the command signal 48 increases, the actuator will move toward its defined closed position. Likewise, when the command signal 48 decreases, the actuator will move toward open. Since mechanical couplings and mountings are not easily altered, reverse acting is better achieved by rewiring the motor 10 and feedback potentiometer 42 connections to obtain the desired relationship. While this polarity-sensitive relationship is quite simple, it is a source of common problems since there are multiple ways to invert the relationship.
When the positioner 40 turns off a given motor winding, inertia built up in the motor 10 allows the motor 10 to continue moving past the desired position. When this occurs, the positioner 40 will immediately attempt to turn on the opposite motor winding to reposition the actuator. Inertia in the opposite direction then causes the actuator to coast past the desired position again. The end result is that the positioner 40 is never satisfied, and the actuator will oscillate back and forth. This is referred to as “hunting”. To avoid hunting, conventional positioners employ a deadband adjustment, which effectively adds offsets to comparators 50 and 52 to create a third state where both motor windings are off. This results in a condition that requires the difference between the command signal 48 and the feedback signal 46 to be greater than the deadband setting before the motor 10 can be turned on again.
The coasting effect of a motor can vary widely depending on actuator speed, motor size, the type of valve used, loading on the valve, and environmental conditions. For commonly available actuators, the coasting effect is at least 0.5° and can be as much as 30°. To minimize coasting, many actuators employ a mechanical brake. While a wide variety of brake designs are implemented, most utilize a common design principle. The brake consists of some type of mechanical device that applies friction to the motor shaft and some type of solenoid device that releases the mechanical friction from the motor shaft. By connecting the solenoid device across the open and close motor windings, the solenoid will release the brake anytime either motor winding is turned on. Likewise, when both windings are turned off (in the deadband range), the mechanical device applies the braking friction to the motor shaft. Since a mechanical brake involves moving parts that require time to move, they are not effective for reducing the coast below 0.5°. However, mechanical brakes are effective for limiting the coast to 2° or less. Mechanical brakes serve a second purpose of holding the actuator's position after a positioner turns off the motor. Actuators that implement spur gear designs are easily backdriven by loads on the valve and consequently almost always employ a mechanical brake.
Since mechanical brakes work on the principle of friction, brake performance will vary with temperature and wear. Additionally, materials used in brakes quite often cannot withstand the higher temperatures that may result if the motor overheats. The solenoid device in the brake is also prone to temperature, where the solenoid may not disengage the brake, thus resulting in eventual overheating of the motor, causing permanent damage to the brake. To eliminate the mechanical brake, some actuator designs utilize mechanical techniques (such as a worm gear drive) that prevents backdrive from the load. However, this does nothing to eliminate coasting that effects the performance of a positioner since the source of the inertial energy, the motor, is not arrested.
Resolution
In relation to actuators, resolution is a measure of the smallest repeatable motion that can be made. For quarter turn actuators, resolution is measured in degrees of rotation. Since the ultimate purpose of using electronic positioners is to control a valve to a specific percent-open position, performance of a positioner is measured according to its ability to consistently achieve any desired or commanded position. Therefore, resolution becomes the most significant measure of performance.
The primary factor of an automated control system affected by resolution is the number of discrete motions the actuator can make over a given control range. As mentioned earlier, conventional positioners employ a deadband setting to prevent unstable operation or hunting. Since the motor is intentionally turned off until the difference between the command signal and feedback signal exceeds the deadband setting, the deadband setting dictates the smallest discrete motion that can be made. As previously mentioned, the mechanical characteristics of an actuator results in a deadband greater than 0.5°, or 180 points of resolution over the 90°. While combining certain positioners with certain actuators can optimize resolution, achieving more than 200 points of resolution is not practical without employing new technology or techniques.
While 180 points of resolution may appear to be significant, commonly used butterfly valves and ball valves can only make use of about ⅓ of the available resolution. This is due to the fact that these types of valves essentially allow 0% to 100% of their flow by the time the valve reaches a point of 33% open, thus reducing the usable points of resolution to sixty. Since most automated systems try to control a particular flow rate, the process controller typically operates a valve in an even narrower range that does not usually exceed 10°. This results in only twenty points of resolution to maintain a stable flow.
To achieve a particular flow rate through a valve, a process controller may move a valve between two points of resolution in order to achieve a “point” in between. Due to the damping effect of large volumes of fluids or gases, switching between two points of control fast enough results in a controlled flow that is an average point between the two points of resolution. Often, the rate at which a process controller needs to switch between two points is faster than the actuator motor can operate without overheating. To compensate, a more expensive actuator with a higher duty cycle is used.
One technique used to improve resolution in special applications is to use precision machined ball valves known as V-ball valves. This type of valve makes use of nearly the full range of the valve by restricting the flow through the valve with a precision cut ball that is shaped like a “V”. This allows the full 180 points of resolution to be used from 0% to 100% open. The net result allows the process controller to use a wider motion, or more points of resolution, to maintain a stable flow.
Another technique used to improve resolution is to expand the actuator's range from 90° to 180°. By gearing the actuator output shaft back down to a 90° motion for a valve, the resulting resolution at the valve can theoretically be reduced in half to 0.25°. However, each stage of gearing introduces backlash that cannot be compensated for since the positioner can only monitor the position of the actuator's output shaft. This results in a practical resolution of 0.3° or more. While the technique of expanding the actuator's range could be further expanded, the increased backlash produced by the gearing between the actuator and valve puts a practical limit on resolution of 0.3°.
In order to achieve better resolution using mechanical techniques, another practical limitation encountered is cost. By design, an actuator achieves better resolution when it employs a mechanical brake, is geared to a slower speed, and utilizes one of the techniques mentioned above. In practice the end result is that higher resolution is obtained at a significant cost for larger, slower, and bulkier actuators being used with more expensive specialized valves.